Generally, a CVVT (Continuously Variable Valve Timing) has been applied to a vehicle in order to reduce exhaust emissions and improve fuel efficiency and power output. Recently, the development of an intermediate phase CVVT system is being pursued by recognizing the limits to the responsibility and work area limitation of the conventional CVVT system and in order to improve it. Since this intermediate phase CVVT controls the cam position at an intermediate position, not the most advanced position (intake stroke) or most retarded position (exhaust stroke), the responsibility thereof is rapid and the use area of the cam can be widened, thereby improving fuel efficiency and reducing exhaust gases.
Meanwhile, in a case of a lock pin of the intermediate phase CVVT, the lock pin installed at a rotor side is locked into a lock pin hole formed at an intermediate position between an advance chamber and a retard chamber when the engine RPM (Revolutions Per Minute) is being reduced, thereby preparing for the subsequent engine start-up. At this time, the state where the lock pin is automatically locked into the lock pin hole during the engine RPM is reduced is a so-called “self-lock” state.
The self-lock is the function that allows the CVVT to return to an exact position mechanically without further adjustment in order to maintain driving safety of the engine at an area other than the operation area of the CVVT, that is, an idle driving state or a start-up of a vehicle.
However, when the CVVT is self-locked, it is possible to secure the intermediate position in the related art by balancing oil pressures of the advance chamber and the retard chamber. However, as shown in FIGS. 1 and 2, the oil pressure balance of the advance chamber and the retard chamber is broken by the average cam torque occurred. Specifically, since the average cam torque is strongly directed always toward the retard direction, a problem exists that the intermediate phase cannot be secured and an offset Θ occurred.
The foregoing is intended merely to aid in the understanding of the background of the present disclosure, and is not intended to mean that the present disclosure falls within the purview of the related art that is already known to those skilled in the art.